Currently, a noble metal particle is used widely as a catalyst. One of its applications is a catalyst for a fuel cell electrode. Fuel cells are roughly classified by the kind of electrolyte they employ: a fuel cell that operates at a lower temperature, such as a polymer electrolyte fuel cell, an alkaline fuel cell and a phosphoric acid fuel cell; and a fuel cell that operates at a higher temperature, such as a solid oxide fuel cell and a molten carbonate fuel cell. Among them, a polymer electrolyte fuel cell (PEFC), which operates at a lower temperature and has high energy conversion efficiency, is one of the most promising power sources for a high-efficiency power generation system and a next-generation automobile. In particular, a direct methanol fuel cell (DMFC), a kind of PEFC, is compact due to its direct use of methanol as a fuel source and being free from a hydrogen storage tank and a reformer. In addition, this type of fuel cells starts up quickly. Therefore, it is expected as a power source for mobile devices.
A polymer electrolyte fuel cell typified by a DMFC has a structure in which an electrolyte member is sandwiched between an anode and a cathode. A noble metal particle as a catalyst is fixed on the anode and cathode. Platinum mainly is used as such a noble metal.
Hydrogen gas fueled into the anode is decomposed into protons and electrons by a platinum catalyst. Hydrogen supplied to the anode is obtained by reforming hydrocarbon-based fuel such as methane or hydrocarbon-oxide-based fuel such as methanol.
This reformed gas, however, contains a tiny amount of carbon monoxide. Strong bonding of carbon monoxide to adsorption sites or active sites on the surface of a platinum catalyst may decrease the catalytic effect or stop the catalytic activity, causing the deterioration of the cell performance. This is called catalyst poisoning. In order to prevent catalyst poisoning, a platinum-ruthenium alloy catalyst has attracted attention. Ruthenium is a hydrophilic substance on which hydroxyl groups are likely to be adsorbed. It is believed that ruthenium bonded with hydroxyl groups oxidizes to remove carbon monoxide adsorbed on the surface of platinum, enabling a platinum-ruthenium alloy catalyst to prevent the reduction in performance of a platinum catalyst. It has been supposed, however, that platinum and ruthenium can provide excellent catalytic properties only when they are used as an alloy.
A platinum-ruthenium alloy catalyst has been produced by precipitating platinum and ruthenium one by one or all at once and then burning them at a high temperature (for example, at 180° C. or higher) in an inert gas or a hydrogen gas (see, for example, JP 09 (1997)-153366 A and JP 2002-102699 A).
However, there is a problem that burning at a high temperature for alloying makes catalyst particle size larger and reduces the surface area per unit mass of the catalyst, which causes the reduction in catalytic properties. In addition, a conventional noble metal particle is produced so that the particle is covered with a protective colloid. Therefore, this protective colloid has to be removed by high-temperature burning in order to use the particle as a catalyst, which causes another problem that this high-temperature burning makes the catalyst particle size still larger.
In consideration of the above-mentioned situation, the present inventors have suggested recently, in WO 2005/030416 A1, a platinum-ruthenium-based noble metal particle that is produced while preventing the catalyst particle size from becoming larger without the need for high-temperature burning for production. In this noble metal particle, ruthenium-platinum alloy particles are deposited on the surface of a platinum particle.
According to the conventional noble metal particle disclosed in WO 2005/030416 A1, the reduction in the catalyst properties can be inhibited by preventing the particle size from becoming larger. However, there is still room for improvement in its methanol-oxidation property.